@GetMapping("/detail/{id}") public String getDetails(@PathVariable Long id, Model model) { Optional bookOptional = bookService.getById(id); if (bookOptional.isPresent()) { Book book = bookOptional.get(); model.addAttribute("book", book); // 평균 평점을 계산하여 모델에 추가 double averageRating = book.getReviews().stream().mapToInt(Review::getRating)..